Fishless system?

[Ruth]

This might sound like a stupid question but I can't remember ever seeing in asked or answered. If you have a system with no fish - only corals and clams - what would be the effects of this - so no fish - no feeding - no fish waste. Would you have to suppliment the system with an additive of some sort like Phyto for softies and some LPS. What about if you only have SPS? Are just regular water changes and maintaining good water quality enough for these corals to get what they need to grow and thrive?
I know I should probably be able to figure this out but have never had a system with no fish. The only inverts would probably be some snails although I may consider an anemone.